Aircraft drift indicator



.Sept 23, 1958 W. L.. CHERRY, JR l2,853,700

AIRCRAFT DRIFT INDICATOR Filed July 6. 1949 4 Sheets-Sheet 1 wim] /F f uE INVENTOR.

waffe z., wf e//e .1 BY w Sept. 23, 1958 w. L. CHERRY, JR

AIRCRAFT DRIFT INDICATOR 4 Sheets-Sheet 2 Filed Jyly 6. 1949 4Sheets-Sheet 3 W. L. CHERRY, JR

AIRCRAFT DRIFT INDICATOR lul 1N VEN TOR Wzfe z. (//fee/ Je. BY

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46E/Vf*- Sept. 23, 1958 Fjiled July 6, 1949 Sept- 23, 1958 V w.LACHERRY, JR 2,853,700

AIRCRAFT DRIFI INDICATOR Filed July s, 1949 4 sheets-sheet 4 EIB A Am,i. v Amy United States Patent AIRCRAFT DRIFT INDICATOR Walter L. Cherry,Jr., Northbrook, Ill. Application July 6, 1949, Serial No. 103,281 4Claims. (Cl. 343-7) (Granted under Title 35, U. S. Code (1952), sec.266) l direction with the airplanes apparent direction, as found byordinary compass, the angle of drift is constantly made known to thenavigator.

It is a further purpose of the invention to provide an instrument suchas described above that operates automatically and continuously, with noattention from the navigator, irrespective of visibility of the earth orsky.

A simple means for indicating the actual direction of flight comprises,in accordance with the invention, a compass 1n which the needle isdriven from a master compass vin the usual manner. In place of theconventional compass card, however, is substituted a card similar to thenormal one but free to rotate about the same axis as the needle anddriven by the drift data in such a manner that it has an angulardisplacement from the aircrafts apparent direction equal to thedifference in the angles of apparent direction of flight and actualdirection of flight. Thus the compensation for drift Ibecomes automatic,for the navigator now reads a compass constantly corrected for drift,and his compass reading is the true direction of flight of the aircraftas referred to the earth.

The apparatus for obtaining the drift data utilizes the Dopplerfrequency change present in high frequency radio waves when there isrelative motion between the transmitter-receiver and a reecting object,which in this case is the earth. The aircraft carries similartransmitting and receiving antennas designed to have beams as sharp aspossible and to scan conically about a vertical axis. The scanning ofthe two antennas is synchronized so that the angle in the horizontalplane between the scanning beam and the longitudinal axis of theairplane is always the same for both antennas. The transmitting antennaradiates a high frequency wave and the reflections of this waveintercepted by the receiving antenna vary in frequency, due to theDoppler effect, froma maximum value when the antenna beams are pointingin the direction of travel of the airplane to a minimum value when thebeams are pointing 180 away from the direction of travel of theairplane.` This frequency variation is sinusoidal and has :a periodequal to the scanning period. The received frequency modulated wavedescribed above is converted by means of a limited and discriminator inthe receiver into :a sinusoidal voltage wave of the scanning frequency.By comparing the phase of this voltage with that of the scan- .ningcycle, referenced to the longitudinal axis of the aircraft, informationas to the angle between the true and apparent directions of the airplaneis obtained. A servocontrol system capable of making this comparison isprovided and operates to keep the compass card rotated from its no-driftposition through an angle equal in magnitude and opposite in sense tothe angle of drift. ,The compass ice therefore directly reads the truedirection of ight with reference to the earth.

The specic details of asuitable apparatus utilizing the principles ofthe invention will be described in connection with the accompanyingdrawings in which Fig. 1 illustrates the type of indicator employed;

Figs. 2a and 2b show the scanning pattern of the antennas;

Fig. 3 illustrates the relationship Ibetween the horizontal velocity ofthe airplane and its radial velocity toward the earth in the directionof the antenna beams;

Fig. 4 shows the Doppler frequency variation with respect to the angleof the antennas;

Figs. 5a and 5b show the complete circuit diagram of the apparatus; and

Fig. 6 illustrates the operation of the servo-control system used in theapparatus.

Referring to Fig. l the indicator employed comprises a compass card 200rotatable about the axis of needle 201. The needle may be driven from amaster compass of any type and by any suitable mechanism to indicate themagnetic or true heading of the aircraft when the card is in its zero orno-drift position. A gear 202 engages the periphery of card 200 and isdriven by shaft 203 which is in turn driven by the drift servo. As willbe shown later, the drift servo operates through shaft 203 and gear 202to rotate card 200 from its 11o-drift position through an angle thatalways equals the drift angle and is in opposite direction thereto. Forexample, when the airplane is drifting 2 to the right of its apparentcourse, the compass card will assume a position 2 counterclockwise toits normal no-drift position making the compass reading a true readingof direction.

The basic principle involved s one of radio echo, and differs from moreconventional radar in that it employs a continuous wave undergoing theDoppler effect due to the motion of the plane, as explained insucceeding paragraphs.

If an oscillator sends out radio waves of sufficiently high frequency, acertain amount of radiation will be reflected back by any object in thepath o-f the waves. If the reflecting body is in motion it will receivea wave not of the outgoing frequency but of a frequency greater or lessthan the outgoing one by the number of wave lengths travelled by thereflecting body in a radial direction from the transmit- 1 ter in onesecond. If the reflector is moving toward the transmitter, the receivedfrequency will be greater than the transmitted one; if the reector ismoving away, the returned frequency will be less than the transmittedone by the same amount.

The reflector now sends back a portion of the received radiation towardsthe transmitter. Inasmuch as it is moving, it produces a second Dopplerfrequency shift as a virtual transmitter equal in degree and kind, tothat produced by it as a receiver. Hence, by the time the original wavereturns to its source, it has undergone a double Doppler shift.

This may be stated by the following equation:

FD=F:I:2V1F

F=original transmitted frequency V1=component of velocity, V, movingradially from transmitter :speed of light FD=reected frequency Forexample, if the wave length of the transmitter is one foot, and if thereflector is moving toward the transmitter at the rate of 500 feet persecond, the increase in frequency, or the Doppler frequency, will be 2500 cycles per second or 1,000 cycles per second. It will be observedthat, irrespective of other considerations, the higher the frequencyemployed, the' greater the Doppler shift, and hence the more superiorspeed discrimination may be obtained.

Now if the transmitter is moving and the reflector standing still,suchas is the case when the `transmitter is mounted in an airplane, theearth becomes a source o f Doppler reflections. Further, if the originalwave be transmitted as a narrow beam toward the earth, and this beam hasa horizontal component in the direction of ight of the plane, theresultant Doppler shift will be a function of the planes speed asreferred to the earth.

In this invention the above principle is applied for determiningtheactual direction of flight of Man airplane,

It should be mentioned here that extremely high Ifrequenciesaremostdesirable for use in th,issystem. The S band represents probablythe lowest frequency usable. The X or K bands should be more desirablefrom a standpoint of mechanical, Doppler and electrical efficiency.

Similar transmitting and receiving antenna assemblies are to be mountedin the belly of the plane in such a manner that the axes of theirparabolic reflectors are perpendicular to the plane of level flight.Each antenna with its reflector produces a conical beam with an angle 9.Further, each antenna within the reflector is rotated or wobbledsynchronously with the other about the axis of its respectiveparaboloid, causing its radiation conical beam to `scan the earth at 30cycles/sec., generating a cone of revolution of degrees, as measured`from the axis of the cone of the radiated beam of angle 0.

Fig. 2a is a diagram of a paraboloid representing either thetransmitting orreceiving reflector whose axis is vertical to the earthsplane, and whose antenna is being rotated to generate a cone ofrevo-lution, whose interception with the earths plane benerates thedotted circle as shown, the radius of which is g r= a tan o Fig. 2.a isa hypothetical case, for it portrays a tilamentary radiated beam, where:0, and would be the ideal case which can only be approached and neverattained.

Fig. 2b represents the dynamic radiation patternvac-l tually produced,where H has a nite value which is determined largely as follows:

o: K paraboloid diameter transmitted frequency The two antennaassemblies are electrically and mechanically identical, with similarangular phasing and polarization and synchronously rotate-d about theirrespective paraboloid axes in order to maintain at all times their phaseand polarization relationship as will be seen more clearly in Fig. 5a.The antennas are mounted close enough to each other in the airplane,that their dynamic radiation patterns are identical for practicalpurposes, considering the normal altitude an airplane maintains.

The desirability of minimum obtainable beam width becomes evident whenit is remembered that the reflected` for lamentary radiation.

But for actual conditions there are an innte number of V's within theradiation cone oft?. For each filament in fro-nt of or behind the centerfilament of the cone by an angle such that the difference in V1 is equalto one transmitted wave length, the Doppler frequency will increase ordecrease one cycle per second.

The situation is further complicated by the fact that the radiated beamwidth 6 is taken as the width of the beam at its half-power point. Hencethe intensity of the various Doppler radiations will also vary from thecenter of the beam to its outermost extremities and in a mannerdependent upon the field strength pattern of the antenna assemblies.This pattern `is largely a function of wave length to paraboloiddiameter.

This latter statement at first hand would indicate that for this reasonalone a shorter wave length would be predominantly superior than one,for example, twiceas large. This is not entirely true however for anygiven permissible size of paraboloid, for, although as the wave` lengthis decreased the beam width 0 is reduced, the difference of V1 requiredfor a change of radial speed of one wave length per second is Vequallyreduced, inasmuch as the sine closely approximates a linear function ofthe angle when the angle is small.

The optimum `angle of the cone of revolution is determined principallyby two factors.

First, the angle must be small enough to permit sufcient reflection to`be `directed back `fromthe earth to the transmitter to produce`adequate received signal. lf the earth were a `perfect plane reflectingsurface, there would be rellection only if qs equalled zero. inasmuch asthe earth is -farifrom a perfect plane as regards wave lengths i-n theorder of l0 cm., .some reection back to the transmitter will occur ifthis or shorter `wave lengths are employed.

Second, `the angle qa must be large enough to provide a radial velocitycomponent V1 great enough to give a reasonably hghDoppler frequency,such that the difference between maximum Doppler frequency and minimumDoppler frequency (the antennas pointed in direction of forward-motionof airplane Vs. pointing in direction `180 to forward) gives goodfrequency discrimination.

This latter statement is the most important single rea son `for using`the highest frequency possible compatible with power outputs ofavailable oscillators.

Now, if the two antennas are rotated synchronously as described, and arephased so that each radiates in the same direction at any instant, theywill sweep the earth in such a manner that their Vl varies sinusoidallyas the antennas point rst in .the direction of flight, sweep to olfilight direction, then opposite to fiight direction, then 90 on theother side of tlight direction, etc.

This variation in V1 produces a similar variation in returned echofrequency FD since where gb is the constant `angle previously selected,and is the angle the antennas makeyat any instant withithe direction offlight. Hence, letting The important feature of the above is that V1 hasa maximum value when the antennas are pointed in the direction offlight, `which is not necessarily the direction the plane is-pointing.These two directions will of course, be identical only :when there `isno drift of the airplane. This is the basisof operation of theinstrument.

4 gives an approximation of the echo frequency vs. relationship. What itdoes not portray is-tl1e spuriousfrequencies dueto finite beam width.However, it does portray in a general way the net result as a sinusoidalvariation of resultant Doppler frequencies.

The `complete circuit diagram of an instrument for indicating truedirection of flight in accordance with the invention is shown in Figs. aand 5 b.

Referring to Fig. 5a, there is provided -a power oscillator V1 of thevelocity modulated type and a first local oscillator V2 of the crystalcontrolled type. The first local oscillator is designed to produce afrequency FLl which is applied through secondary winding L23 to thecontrol grid of power oscillator V1 to frequency modulate this tube. TheNo. 1 and No. 2 cavities of V1 are designed to resonate at frequency F,the transmitted frequency. Cavity No. 3 is 4designed to resonate atfrequency F-FLL Hence the output of cavityv No. 2 is predominately F andthe output of cavity No. 3 pr'edominately F-FLI.

Radio frequency energy of frequency F from cavity No. 2 of V1 is appliedthrough rotating coupling CT to transmitting antenna T which is housedin its paraboloid assembly. Reected radio frequency energy of frequencyKV eos is picked up by receiving antenna R, housed in its paraboloid.Both T and R, produce beams 0 degrees wide, are electrically eccentricby an angle and free to rotate because of rotating couplings CT and CR.The reliected energy received by R is fed Viacoaxial cable and rotatingjoint CR to crystal lmixer M.

The output of cavity No. 3 of V1 is fed Via coaxial cable to filter No.4 composed of two cavities tuned to F-FLl. The output of this filter isvirtually entirely F-FLl, and is fed to the crystal mixer M where it ismixed with signal from antenna R.

The mixing process carried out by this crystal produces the sum anddifference frequencies of its input signals. For the differencefrequency The sum frequencies are so high as to be virtually allattenuated inl the mixer cavity 210. The output of the mixer thereforeis a first intermediate frequency which FLl, which may be termed thefirst intermediate frequency carrier, should preferably be acomparatively high frequency such, for example, as 30 megacycles.

There is a leakage from antenna T to antenna R of a small portion of thetransmitted energy, due to the proximity of the antennas, and theirrespective side lobes. Mixing of this with the received Doppler signalin crystal mixer M would result in amplitude modulation of the firstintermediate frequency of a spurious nature, and must be eliminated.This is done by feeding some of the energy from V1 into the coaxial linefeeding the mixer M of such a phase asto be in exact phase opposition tothe leakage energy, and of such amplitude to just neutralize the leakageenergy. The correct phase of this counter feed is secured bycorrectplacement of the counter feed input into the mixer coaxial feeder 211,and the correct amplitude is obtained by means of cavity attenuator A.

afasaf The two antennas are phase and synchronously driven, Y

KVcos C is fed from the crystal mixer M through terminal 212 to the gridof the iirst I. F. amplifier V3, Fig. 5b.

V3 is a conventional high frequency I. F. amplifier with tank circuitC31-L31, cathode bias resistor R31 and by-pass condenser C32, screen andplate decoupling resistor R32 with decoupling condensers C33 and C34,and C35 coupling condenser to the following I. F. amplifier stage V4.The input from crystal mixer M is fed to the grid of V3. The D. C. gridreturn and path for rectified crystal current is through choke L32 viameter I to ground. C36 protects the meter from any leakage R. F.current. Meter I thus gives an indication of the proper functioning andfeed of first local oscillator V2, plus a rough indication of mixercrystal eiiiciencv.

V4 represents the second stage of I. F. amplification, and is identicalin function to V3, with the exception of the input, via couplingcondenser C35, and grid return resistor R43. C41-L41 represents the tankcircuit, R41 the cathode bias resistor and C42 the cathode by-passcondenser. R42, together with C43 and C44, form a screen and platedecoupling filter.

V5 is the third I. F. amplifier stage, and is identical F-FLI -lyto thesecond I. F. `amplifier s't-age. C45 and R53 comprise the coupling fromV4 while CS1- L51 comprise the tank circuit. R51 is the cathode biasresistor, and C52 is cathode by-pass. R52, together with C53 and C54,form the plate and screen decoupling filter.

At this point it is necessary to heterodyne the first intermediatefrequency with a second local oscillator. This is necessary because thepercent devi-ation from FL1, `the first l. F. carrier, as caused by theDoppler signal is insufiicient to givegood discrimination. It will berecalled that FLl was chosen relatively high (in the neighborhood of 30megacycles) in order to obtain good I. F. amplifier characteristics.

V6 is a combined second crystal controlled local oscillator and mixercomprised of crystal X2 and plate tank circuit C67-L62. Grid No. 2 of V6acts as a plate in this circuit. R65 is the oscillator grid leak, R61the cathode bias resistor and C62 the cathode by-pass. The output ofabove described oscillator section of V6 is electron coupled to theremaining section of V6 where it is mixed with the output of V5. C55 andR63 form the input coupling from V5 to this mixer sections control gridKV cos FLl-l- C Now, if the second local oscillator frequency PL2 ismixed with this input, the sum `and difference frequencies appear in theoutput of V6. In this case, tank circuit C61L61 produces a selectivefiltering action so that for practical purposes only the differencefrequency is of sufciently great amplitude to be useful. 'Thisdifference frequency is and may be termed the second intermediatefrequency.

It consists of a carrier frequency FL1-FL2 about which varies theDoppler variation f f KV cos ,s C

The output of V6 is inductively coupled to the input of V7 via parallelresonant circuit L71-C71. V7is an amplifier limiter, an'overdriven,under-biased, variable mu pentode, whose purpose isto remove allamplitude rmodulation fromy the signal and` prepare it for the followingdiscriminator V8. Amplitude modulation at this R71 is a grid currentlimiting resistor which biases V7 because of grid current owingin it.,The higher the sig,- nal input, the more grid current is drawn, thehigher the bias generated and theflower the amplification produced inthe tube. C73 is the screen by-,pass to ground, and

,R72 is the screen dropping resistor. C75 is a decoupling:

. condenser.

V8 is a conventional frequency modulation discriminator. The secondintermediate frequency is fed from of Ianode current.

kfor the conditions (a) zero Doppler phase shift (no l half of V10.

- point would represent distortionof the signal, and hence f wouldintroduce error in the discriminator output.

Each cathode of kV10 is coupled yia resistors R122 and R132 to ayrespective gridof triodes, V12 and V13, and

the potential developed at the cathodes of V10 helps form kthe biasof'tubes V12 and V13.l

yV12 andV13 are power ampliers which are normally balanced to give equaloutput by potentiometer R125; as fed by voltage divider R123, R124,when` their grids receive no input.

the plate of V7 to tank circuit L72-C72. `Tank circuit i LS1-C81iszinductively coupled to L72-C72, and` also` capacitively coupled jtoL72-C72 via C74. LSL-C81 is tuned tto the carrier frequency of thesecond. in terme#y diate frequency, FL1---FL2-` R81, R82 are theiload;resistors for the two `diodes of V8and the mid-tap1of`R81 also serves asa direct current rreturn to the midrtap; of L81, completing the D. C.diode circuit.` C82 is the filter cycle` wave of the referencegeneratonwill cause one-y condenser lfor the output of` thediscrirninator, and1 by.

n passes the high frequency components of the second I,e F.,

leaving as the discriminator output only the, 30` cycle current producedby the 30'cycle sweep of the` antennas, and their received Dopplersignal.

The output of V8 is .coupled via C91 to-.thelrst grid,`

as a cathode follower in order` to present a high ,andiireasonablykconstant load. impedance to discriminatory V8.

R91 is the grid leak, while R92 is the cathodefloadfe sistor.

The rst half of V9 is coupledto the grid of the second half Via C92.This second half is an overdriven, underbi-ased audio amplier` withcathode bias resistor, R93, cathode by-pass C93, grid lead R94 and.plate load resistor R95. The output from the second half of, V9 is a 30cycle square wave, due to the limiting action of this section of V9. The30 cycle square wave is` fed to parallel grids of the double triode`commutator tube V10 via condenser C94.`

The plates of V10 are connected push-pull to the plates of V11. Tube V11is an underbiased overdriven double triode used as a squaring amplifier.Its input is a sinusoid produced by single phase` reference generator.G1 (Fig. 5a), which is connected via gearing GS tothe motorthat rotatesthe two antennas T and R. G1 is vdriven at the same speed as theantennas, and hence its output sinusoid is always in synchronism, butnot necessarily. in phase, with the 30 cycle input to V10, asproducedby`the Doppler eiect.

Transformer T1 produces 180 degree phase inversion of its input from G1in order to feed tube V11 in pushpull. R111 and R112 are grid currentlimiting resistors, and R114 and R115 are plate load resistors. Theplate outputs ofeach half of V11 are 30 cycle square waves, and theoutputs are180 degrees out of-phase with eachff other.

Theftwo parallelV grids of tube V10 are biased-to cut off for any platevoltage they may be subjected to by means ofbattery, BA10, coupled viaR101.

It is the matter of phase relationship between the reference generatorsquare wave and the Doppler-produced square wave which gives this systemits drift sense. Referring to Fig. 6, this tigure shows the voltages onthe plates, grids and cathodes of both sections of tubeVlO of doubletriodel V9. The rst half of V9 is connected Under the conditions. shownat Fig. 6 (a), their out` put will still be balanced, and hence thesplit iield servomotor B2 connected in the plate leads of V12l and V13will not rotate, because its' opposing elds are energized an equalamount. n

it ywill be seen that any change in this balanced phase relationshipwill cause a4 diierent current to flow in the two sides of V10. A phaseshiftin either direction of the yDoppler-produced 30 cycle wave withrespect to the half of V10 to conduct more than thek other. The half ofyV10 thatvconducts theiwave current depends upon kthe direction of thephase shift; yConditions of unbalance in V10 are portrayed in Fig. Gat(b) andtc).

rAny unbalance in the output of V10 will cause an unbalance in theservo'motor iield, and the motor will rotate in one direction ortheother, as determined by the direction of unbalance. y

This servomotor rotates, by means of coupling 215 and a gear train216-'217 (Fig. 5) the iield coils only yof, the reference generator G1in such a direction as to bring about a phase relationship between itsoutput .and

the 30 cycle Doppler-produced output such that V10 is balanced oncemore. When this has occurred, theentire system is balanced and nofurther rotation of the servomotor B2 occurs.

The reference. generator must therefore havea somewhat unconventionalmounting. Its armature rotates in the normal manner, but instead of itseld and shell being mounted rigidly, there is a gear atlixed to theshell 218 which changes the angular location of the elds with respect tothe armature, and hence the antennas,` at any instant. In order toinitially set the generator phase so as to obtain the balanced conditionof Fig. 6 (a) when there is zero drift, provision is made to rotatablyadjust gear 217 with respect to shell 218. After proper initialadjustment has been made the gear 217 may be locked to shell 218 byslotted ange 219 and screw 220.

The same shaft that drives the shell of G1, also drives, through gear202 (Figs. 1 and 5), the graduated compass dial, as described before.

The system is aligned so that` a balanced output of V10 exists whenthere is zero drift of the airplane. That is, the square Waves producedfrom the output of the 30 cycle reference generator and the 30 cycleDopplergenerated signal are as shown in Fig. 6 (a). Now, if the airplanestarts to drift, there will be a relative phase shift between these two30 cycle signals, inasmuch as the reference phase remains the same (i.e., that for ight along the axis of the plane,l *with no drift) but theDoppler-produced signal now changes from a maximum whenthe antennas. arepointed along the major axis `of the airplane to a maximum when theantennas are pointed in. a direction to the left or right ofV the majoraxis, because of the direction of maximum speed` ofthe airplane withrespect to the earth having changed due to drift. This situation existsonly momentarily, as the servo system operates to align the phase ofthese two signals to produce balance at the output of V10 once more.

G2 is a D. C. anti-hunt generator, and C121, C131, and R134 serve todifferentiate its output and feed it to the grids of V12 and V13 viaresistors R126 and R133. When servomotor B2 and anti-hunt generator G2are running at constant speed no voltage is developed across resistorR134. However when the field energization of B2 is suddenly reduced tozero by tube V10 becoming lbalanced and B2 begins to lose speed, avoltage is developed across R134 in proper polarity to apply, throughits action on the grids of tubes V12 and V13, a momentary reverseenergization to motor B2 to bring it to a quick stop.

The time constant of the entire servo system should belong enough tooverride normal variations in Doppler echo produced by roll of theairplane, 4variations in terrain etc.

The power supplies shown in Figs. 5a and 5b are meant to be illustrativeonly. VRI is a disc type rectifier fed by transformer TP1 to providedirect current for the filament of V1. LPI, LP2, CP1, CP2, form thesmoothing filter, and RF1 the bleeder for this supply. C12, C13, C14,C15, L11, L12, form a decoupling filter to prevent radio frequencyenergy from being fed back to this power supply. VP1 is the high voltagecathode supply rectifier fed by TP2. Its output is smoothed by lter CP3,CP4, LP3. VP2 is the plate voltage rectifier fed by TF3, which suppliesplate current for the rrest of the equipment. Its output is smoothed bylter LP4, LPS, CPS, CP6. RP2 is a bleeder resistor.

What I claim is:

1. A device for determining the angle of drift of an airplane, saiddevice comprising means on the airplane for generating a sharp beam ofhigh frequency radiant energy having a fixed angle with respect to thevertical, means for rotating said beam about a vertical axis at constantangular velocity, means on said airplane for receiving reflections ofsaid energy from the earth, the frequency of said received energyvarying sinusoidally between a maximum value when said beam is pointingin the direction of flight of said airplane and a minimum value whensaid beam is pointing in a direction opposite to the direction offlight, said sinusoidal frequency variation having a period equal to theperiod of rotation of said beam, means for deriving from said receivedenergy a sinusoidal voltage having the same frequency as said sinusoidalfrequency variation and having a fixed phase relative thereto, and meansfor comparing the phase of said derived voltage with the phase of therotational cycle of said beam to determine the magnitude and directionof the angle between the apparent and realv directions of flight of theairplane.

2. A drift meter for airplanes comprising a pair of similar highlydirective antennas mounted on the airplane and arranged to scanconically and in the same phase about vertical axes, a transmittercoupled to one of the antennas for continuously applying high frequencyenergy of constant frequency thereto, a receiver coupled to the otherantenna for continuously receiving reflections of said high frequencyfrom the earth, said received energy being frequency modulated as aresult of the Doppler effect and the conical scanning at a modulatingfrequency equal to the scanning frequency, means in said receiver forderiving from said frequency modulated energy an alternating voltagewave of said modulating frequency, and means for comparing the phase ofsaid derived voltage with the phase of the scanning cycle of said beamsto determine the magnitude and direction of the angle between theapparent and real direction of ight of the airplane.

3. Apparatusas claimed in claim 2 in which saidcomparing means comprisesa generator for generating a single phase alternating reference voltageof the scanning 1o frequency and synchronized therewith, means forshifting the phase of said reference voltage, means for initiallysetting said phase shifting means so that said reference voltage has apredetermined phase relation to said derived voltage in the absence ofdrift, means coupled to said phase shifting means and responsive to adeparture of said reference and derived voltages from said predeterminedphase relationship to restore said relationship through adjustment ofsaid phase shifting means, and means coupled to said phase shiftingmeans for producing an angular rotation from a no-drift position thatalways equals the angular difference between the instantaneous phase ofthe reference voltage and the phase of the reference voltage when saidphase shifting means has its initial setting, said angular rotationbeing equal to the angle of drift.

4. An instrument for indicating the true direction of ight of anairplane relative to the earth comprising a transmitting antenna and areceiving antenna mounted in close proximity on the airplane, saidantennas being electrically identical and designed to produce a sharpbeam at a fixed angle to the vertical, means for rotating the beams at aconstant rate and in the same phase about vertical axes passing throughthe center of the antennas, means for applying high frequency electricalenergy to said transmitting antenna whereby high frequency radiantenergy is directed thereby toward the earth, receiving means coupled tosaid receiving antenna for receiving reections of said high frequencyradiant energy from the earth, said received energy having a sinusoidalvariation in frequency betweena maximum value when said beams point inthe direction of flight of the airplane and a minimum value when saidbeams point away from the direction of Hight of the airplane, saidsinusoidal frequency variation being of the same period as therotational period of said beams, means in said receiving means forderiving from said received energy a sinusoidal voltage wave of the samefrequency as said sinusoidal frequency variation and having a fixedphase relative thereto, means for generating a single phase alternatingreference voltage of the same period as the rotational period of saidbeams and synchronized with the rotation of said beams, means forshifting the phase of said reference voltage, means for initiallysetting said phase shifting means so that said reference voltage has apredetermined phase relation to said derived voltage when theapparent`and true directions of flight are the same, servo-control meanscoupled to said phase shifting means and responsive to a departure ofsaid reference and derived voltages from said predetermined phaserelationship to restore said relationship through adjustment of saidphase shifting means, a direction indicator comprising a pointer and adial graduated in degrees both rotatable about a common center, meansfor angularly positioning said pointer in accordance with the4 earthsmagnetic field, and coupling means between said dial and said phaseshifting means for producing, in response to adjustment of said phaseshifting means by said servocontrol system, a rotation of said dial froma zero-drift position through an angle that always equals the degrees ofdifference between the phase of the reference voltage at the given timeand the phase of the reference voltage when said phase shifting meanshas its initial setting, whereby said direction indicator always readsthe true direction of ight.

References Cited in the file of this patent UNITED STATES PATENTS

